1. Field of the Invention
The present invention relates to a ground fault protecting apparatus and method for solar power generation and to a solar power generation apparatus using the apparatus and method and, more particularly, to a ground fault protecting apparatus for protecting a solar power generation apparatus from a ground fault generated in a solar cell array and a method therefor, and to a solar power generation apparatus using the apparatus and method.
2. Description of Related Art
A solar cell array has a plurality of solar cells connected in series/parallel and is set outdoors such that each solar cell is sufficiently irradiated with solar rays. If an object, e.g., not only a metal object but also an animal or human erroneously comes into contact with the solar cell and generates a short circuit to the ground potential (ground fault), the solar power generation system incorporating the solar cell array must temporarily stop its operation, from the viewpoint of safety. This stop largely affects whether load is connected to the system.
FIG. 2 is a block diagram showing the arrangement of a solar power generation system connected to a commercial alternating current power system and having a control/protection unit as a measure against a ground fault. In FIG. 2, reference numeral 21 denotes a solar cell array; 22, an inverter; 23, a control/protection unit; 24, a disconnection unit; and 25, a commercial alternating current power system.
In the solar power generation system shown in FIG. 2, direct current power generated by the solar cell array 21 is sent to the commercial alternating current power system 25 through the inverter 22. The disconnection unit 24 having a mechanical contact is inserted between the inverter 22 and the commercial alternating current power system 25. In a system failure, a disconnection operation for electrically disconnecting the commercial alternating current power system 25 from the inverter 22 is performed, and simultaneously, a so-called gate block operation is performed to electronically turn off the switching element in the inverter 22. The control/protection unit 23 detects the states of the commercial alternating current power system 25 and the solar cell array 21 and causes the disconnection operation and the gate block operation to be performed. More specifically, upon detecting the system failure, the control/protection unit 23 sends a gate block signal to the inverter 22 and an operation instruction signal to the disconnection unit 24 to cause the inverter 22 and the disconnection unit 24 to perform the disconnection and gate block operations, respectively.
System failures include, e.g., an abnormality in the commercial alternating current power system 25 and the ground fault of the solar cell array 21. Especially, in the latter failure, i.e., the ground fault, one of the following protecting operations is performed.
(1) First protecting method (instantaneous stop type): the system is instantaneously stopped in response to detection of a ground fault. PA1 (2) Second protecting method (delay stop type): upon detecting a ground fault, the disconnection operation is performed after a predetermined delay time. The delay time is generally set to be 0.5 sec. or less. PA1 (1) The first protecting method, of instantaneously performing the disconnection operation all two readily causes erroneous operation due to noise. For this reason, the ground fault detection sensitivity cannot be very large. If such erroneous operation takes place frequently, the service life of the disconnection unit 24 is shortened, depending on the switching count of the mechanical contact of the disconnection unit 24. PA1 (2) In the second protecting method, of delaying the disconnection operation by a predetermined time, when the delay time is short, erroneous operation occurs readily, and the ground fault detection sensitivity again must be limited, as in the first protecting method. When the delay time is set to be long, the protecting operation itself is delayed to the extent that the protection effect cannot be obtained.
However, these protecting methods have the following disadvantages.